1. Field of the Invention
This invention relates to the field of optical holography and more specifically to improved apparatus and methods of making one-step rainbow holograms.
2. Discussion of Related Art
Rainbow or Benton-type holograms are incoherent light viewable holograms that retain parallax on one axis only, generally the horizontal. Such holograms are described briefly in Benton, S., "Hologram Reconstruction with Extended Incoherent Sources," J. opt. Soc. Am. 59,1545A (1969). A more detailed description can be found in an article by Emmett Leith entitled "White-light Holograms," Sci. Am. 235,80 (1976).
In the prior art, rainbow holograms are commercially made in a two-step process. In the first step a transmission hologram, generally referred to as H1, is made of an object using a standard off-axis reference beam configuration. After processing, this hologram is then illuminated though a narrow horizontal slit by the conjugate of the reference beam used in making H1 so as to reconstruct the pseudoscopic real image of the object from the perspective of the slit. A second hologram, generally referred to as H2, is then made by interposing a recording medium within the plane of this real image reconstruction and bringing in a second reference wave to interfere with the object wave at the recording plane. The resulting hologram reconstructs not only an orthoscopic image of the object but also a real image of the horizontal slit (exit slit) positioned in front of the hologram. When the hologram is illuminated by white light and a viewer is positioned at the exit slit, the viewer will see the entire image of the object in a single hue from the single vertical perspective of the horizontal slit. This hue changes as the viewer changes viewpoint vertically at the exit slit--thus the term `rainbow` hologram.
A drawback of the two-step process is that it requires two completely separate and costly holographic systems to yield the H2 rainbow hologram. Furthermore, the quality of the H2 hologram in the two-step process can be degraded by the limited resolution of the photosensitive medium used to record H1, and its subsequent photographic processing.
In attempts to develop simpler, lower cost alternatives to two-step rainbow holography, several investigators have proposed direct one-step methods. Chen and Yu in an article entitled "One-step Rainbow Hologram," Opt. Lett. Opt. 2,85 (1978) proposed a simple setup with a slit and a single imaging lens that yields an orthoscopic image with the real slit image in the proper position. The horizontal (or holographic) field of view is limited by the size of the imaging lens. Similar optical systems are used in U.S. Pat. No. 4,235,505 and in Chen, et.al. "Generation of Color Images with One-step Rainbow Holograms," Appl. Opt. 17,1490 (1978). One deficiency of one-step lens based systems is the implicitly large and costly imaging lens that would be required to yield a field of view consistent with that of commercial two-step rainbow holograms. A second drawback is the image aberrations that are introduced by the lenses.
A one-step method using a concave mirror has been proposed by Hariharan, et. al., in an article entitled "One-step Multicolor Rainbow Holograms with a Wide Angle of View," Optica Acta 26,289 (1979). The apparatus employed a large diameter concave imaging mirror of large numerical aperture. The object and recording plane were located in line and off-axis at the center of curvature of the mirror, with the slit located between the center of curvature and the focal point. This configuration gives unit magnification, orthoscopic viewing, and proper positioning of the real image of the slit. However, the single mirror does not retain correct left-to-right orientation in the holographic image.
As with lens-only systems, the concave mirror used by Hariharan also introduces image aberrations which even for the large diameter/high numerical aperture mirror chosen, could be of practical significance even for relatively small sized objects. For example, for the mirror chosen, image curvature could be objectionable in critical applications for objects as small as five inches in diameter. Furthermore, by employing off-axis imaging the image sag due to curvature is at least four times as great as with on-axis imaging. As regards field of view, even with the sizeable and fast mirror used by Hariharan, the field of view was only 66 degrees; this does not compare favorably with the 100-120 degree field of view exhibited by the majority of commercial rainbow holograms. Additionally, in-line off-axis imaging makes it difficult if not impossible to uniformly light an object with multiple object beams as is required for high quality rainbow hologram imagery.
In order to be competitive with conventional two-step rainbow holograms, a one-step hologram must have a holographic field of view comparable to two-step holograms. While the field of view in two-step rainbow holography can be increased with little additional system cost and with no effect upon image aberrations, a similar increase for the one-step processes reported in the prior art would necessitate massive, costly optics if image aberrations were to be kept at a minimum. This would obviate against choice of prior art one-step methods.
There is no method in the prior art for choosing the parametric values of the optical system in one-step rainbow holography for a level of acceptable aberrations, image size, and holographic field of view. The present invention satisfies this need.
The ability to vary the magnification of a holographic image in rainbow holography is of interest since it eliminates the necessity for exact one-to-one models. Of particular importance is image reduction since a large, highly detailed model can be made and then holographically reduced to the desired image size. To change magnification in the two-step process requires a large lens located relative to the H1 real image reconstruction. Aside from cost considerations, the lens would introduce aberrations and would need to have a diameter greater than the length of H1 so as not to reduce field of view. In eliminating these difficulties, the one-step apparatus of the present invention makes magnification changes straightforward and requires no additional optical elements. For reduction of the size of the image, the one-step apparatus of the present invention actually increases the holographic field of view. As another benefit, the present invention allows the reduction of conventional H1 holographic images by using the one-step apparatus as the second stage of the conventional two-step process.
One of the primary commercial uses of two-step rainbow holography is in preparation of masters in photoresist for subsequent metallizing for mass hologram replication. Especially for small images such as those used in security applications, direct recording in photoresist in one-step would considerably reduce equipment cost and time requirements. There has been no report of a one-step process for making a rainbow master directly into photoresist, presumably since the required field of view and image quality constraints were not met by the one-step prior art. The present invention satisfies this need.